In a sense, this 31st December deadline isn't too important. Currently the only two things that will stop the company being struck off in February are (1) filing the 2019/2020 accounts that were due on 31st March this year (3-month extension due to Covid), (2) a creditor of the company objecting to the strike off. If she hasn't paid her corporation tax, which runs on a different schedule timings-wise than filing things with Companies House, and HMRC thinks they can get money out of the company, they will object and hold things up.
If neither of those two things happen, the next thing we'll see is a line in the filing history that reads 'Final Gazette: dissolved via compulsory strike-off' - and that's it, the company's gone. Any assets it owned at the time of strike-off along with any cash become the property of the Crown (although I don't think they'll be too interested in taking ownership of any Cotswold Company sideboards purchased through the company as essential staging props).
It's possible there might be an investigation into Jack's behaviour as a director and she could get disqualified for acting as a director for anything up to 15 years, but this is quite unusual. That said, I know three disqualified directors, but I just obviously have dodgy friends! I don't think at Jack's level of turnover she's going to have anything to worry about on that front.
EDIT: Sorry, SundaySchoolTeacher replied while I was typing! Apologies for duplication!